This invention relates to a simplified steering system for track-laying vehicles (i.e., vehicles which use endless tracks rather than tire-covered steerable wheels to contact the terrain over which they are driven, e.g., tractors, tanks, bulldozers, etc.) and for boats and airplanes and, more particularly, to a simplified steering system that permits the vehicle to be steered at all times by a conventional steering actuator without necessitating the use of steered wheels or rudders.
Presently, there is an acute need for a form of track-laying vehicle appropriate for both highway and off-road use over snow-covered, very uneven, or muddy terrain. The need for such a vehicle is great following natural emergencies (snow and wind storms, floods, etc.) and is at present particularly needed in developing countries. Unfortunately, almost all presently available automotive vehicles require infrastructure (paved highways, bridges, etc.) for practical operation, and the developing countries are decades away from having the necessary infrastructure for such conventional vehicles. Further, the only load-carrying off-road vehicles presently in use have either very large wheels or very cumbersome tracks which are heavy, slow moving, damaging to unpaved roads, and inappropriate for use on paved highways. While smaller all-terrain wheeled vehicles are commercially available, these do not carry adequate loads for normal multi-passenger or produce transport, and their drive wheels not only are damaging to unpaved terrain but can also easily become mired in heavy mud or snow.
Our earlier inventions (e.g., U.S. Pat. No. 4,776,235 to V. E. Gleasman et al.) make it possible to steer track-laying vehicles with a single steering wheel in the same manner as other highway vehicles are steered. In these earlier steer-drives, a pair of standard differentials or a pair of standard reduction gear drives (e.g., also see U.S. Pat. No. 4,895,052 to V. E. Gleasman et al.) are interconnected as part of a hydro- (or electro-) mechanical system that is inserted in the vehicle""s drivetrain between the vehicle""s transmission and track propulsion shafts. These steer-drives add and subtract steering torque to and from driving torque that is supplied to each of the respective tracks of the vehicle, thereby permitting the vehicle to be steered with a conventional steering wheel without requiring any simultaneous locking or braking of either one of the tracks.
While the prior art systems just identified above operate satisfactorily, they are relatively large and mechanically complex. Since these steer-drive systems do not replace the conventional engines and transmissions necessary for automotive vehicles, they require additional space and add weight. Therefore, it is commercially desirable to achieve reductions in the size and weight of steer-drives. Such size and weight reductions are primarily limited by the size and strength of the materials required to transfer the vehicle""s driving torque from its engine and transmission through to the tracks.
The maximum torque being transmitted by the various elements positioned throughout the vehicle""s drivetrain is determined by the output of the engine and by any increases in the mechanical advantage of that engine output by virtue of the various gear ratio speed reductions realized throughout the drivetrain. For instance, if the drivetrain includes a mechanical speed-reduction unit that reduces the speed of a given rotational input shaft by a ratio of 2:1, this results in a mechanical advantage that effectively doubles the torque being transmitted, thus requiring a doubling of the strength of the speed-reduction unit""s elements compared to other similar drivetrain elements that normally rotate at higher speeds.
In this regard, standard planetary gear drives (e.g., those shown in several different arrangements in the prior art patents referred to above) provide a limited range of speed-reduced outputs. [NOTE: xe2x80x9cStandard planetary drivesxe2x80x9d consist of a sun gear, an exterior ring gear, and planet gears in mesh with both the sun gear and the ring gear, the planet gears being supported by the arms of a carrier. Such planetary drives can be used in many different ways to provide a variety of positive and negative speed reductions, namely, the input can rotate any one of the components (the sun gear, the ring gear, or the carrier), while output can be taken from either of the other components. However, for practical use in combining steering and driving torque, input is received through the sun gear.] According to automotive textbooks, with input to the sun gear, standard planetary gear drives are all limited in actual practice to producing speed-reduced outputs that range between 2.5:1-5:1. That is, the elements of such planetary gear drives must be made large enough to support the 2.5-to 5-times increase in torque that results from their built-in gear reduction. Therefore, these prior art planetary drives must include elements that are stronger (larger and heavier) than would be needed for components providing lesser speed reductions.
The subject invention was conceived during the testing of a new prototype steer-drive vehicle when trying to develop a further variation of our earlier steer-drive systems that would be lighter and more compact. While the invention uses the same basic steer-drive concepts for combining steering and driving torque, it replaces key elements of prior art steer-drives with a known and relatively simple gear arrangement that, in this new combination, results in a new steer-drive that is not only more compact and lighter in weight but, surprisingly, is also simpler mechanically and reduced in cost. Further, this simpler, lighter, and more compact steer-drive is applicable as well to non-automotive vehicles, namely, boats and airplanes.
In its primary application, this steer-drive is designed for use on tracked vehicles which not only operate off-road but are specifically intended to be driven on paved roads at typical highway speeds. As is well known in the automotive world, most standard vehicles driven at highway speeds normally rotate the axles which directly drive the vehicle""s tires at an overall speed reduction ratio of about 4-5:1 relative to the vehicle""s engine. One of the key objects of the invention is to realize most of this normal overall speed reduction as close as is practical to the drive axles that directly drive the vehicle""s tracks or propellers. That is, the steer-drive of the invention is intentionally and preferably designed to provide a speed ratio (between the input to the steer-drive, i.e., the output of the vehicle""s transmission, and the output of the steer-drive itself) that is as close to 1:1 as is practical without actually matching 1:1.
Prior art steer-drives use pairs of standard differentials or pairs of standard reduction gear drives, the latter being located in proximity to the vehicle""s drive axles. The steer-drive disclosed herein uses neither a combination of standard differentials nor standard reduction gear drives. Instead, the desired combination of driving and steering torque is achieved by respective left and right orbital gear drive units. [NOTE: Orbital gear arrangements (sometimes identified as xe2x80x9creverted epicylic gear trainsxe2x80x9d) are well known, e.g., see U.S. Pat. No. 5,186,692 entitled xe2x80x9cHydromechanical Orbital Transmissionxe2x80x9d issued to V. E. Gleasman et al.]
The orbital gear portions of the steer-drive units disclosed herein are quite simple in format, comprising only an input gear and an output gear interconnected by at least one orbiting cluster gear. As used in this invention, these orbital drive units are preferably designed to transfer driving torque at speed ratios selected to be as close to 1:1 as is practical and, in any event, at ratios less than can be practically achieved with the standard reduction gear drives shown in prior art steer-drives. [NOTE: When used for steer-drive purposes, orbital gear drives must operate at some ratio greater than or less than 1:1. Practical considerations determining the selection of such gear ratios will be discussed in greater detail below, and the disclosed preferred orbital drive units are designed so that, when the additions and subtractions of steering torque are ignored, driving torque is transferred through each unit at a ratio of 1:1.36.]
Each orbital unit has an input gear and an output gear aligned along the same first axis and interconnected by at least one orbiting cluster gear mounted for rotation on an orbit shaft positioned parallel with the first axis. The orbit shaft is supported in a housing that also rotates about the first axis. When rotation of the housing is prevented, the rotation of the input gear drives the cluster gear which, in turn, causes rotation of the output gear (e.g., at a speed ratio of 1.36:1). However, rotation of the housing supporting the orbit shaft causes the cluster gear to orbit about the input and output gears; and even though the speed of rotation of the input gear remains constant, such orbital movement of the cluster gear results in a variation of the speed of the output gear. Thus, rotation of the housing in one direction causes an increase in the speed of the output gear, while rotation of the housing in the opposite direction causes a decrease in the speed of the output gear.
Steering control is achieved by controlling the rotation of the housing of each orbital unit with a simple mechanical arrangement: Namely, rotation of each orbital unit housing is controlled by a steer-drive motor that is responsive to the operation of the vehicle""s steering actuator, the direction and distance of steering actuator motion being translated into a respective variation in the direction and speed of the steer-drive motor. The steering torque created by the steering motor is transferred to the housings of the orbital units by respective rotary connectors, e.g., worms associated with wormgears fixed, respectively, to each of the housings. This steering torque is applied simultaneously, and in opposite directions, to the right and left orbital units; and the steering torque is superimposed over any driving torque being transferred between the input and output gears of each orbital unit. That is, in this simple manner, driving and steering torque are instantaneously combined by each orbital unit. Since this steering torque is applied simultaneously, it causes the propulsion element (e.g., track, propeller, jet) on one side of the vehicle to speed up and the propulsion element on the other side of the vehicle to slow down at the same rate; and this change of speed of the tracks (propellers, jets) relative to each other causes the vehicle to turn in the direction of the slower-moving propulsion element.
[NOTE: Since the rpm of the drive shafts determines the speed and direction of vehicle movement at any given instant, and since that rpm is determined by the loads being carried and the torque being delivered, the terms xe2x80x9cdriving torquexe2x80x9d and xe2x80x9csteering torquexe2x80x9d are often used herein (and in the cited prior art) almost interchangeably with xe2x80x9cdriving rpmxe2x80x9d and xe2x80x9csteering rpmxe2x80x9d, each being intended to refer to the changes in motion necessary for driving and steering the vehicle under prevailing conditions.] When the vehicle engine is operating but no driving rpm is being delivered to the input gears of the two orbital units, steering rpm can still be delivered to the housings of the orbital units by the steer-drive motor. If this is done when the vehicle""s brakes (if any) are not engaged, the vehicle will pivot turn about its center. That is, e.g., in the case of a tracked vehicle, the operation of the steer-drive motor will cause the tracks to move simultaneously in opposite directions at the same speed. Since both tracks are moving, this pivot turn is accomplished without dragging either track and, therefore, with minimal effect on the terrain upon which the vehicle is standing.
In the preferred tracked vehicle embodiment disclosed, each track is driven by a respective front- and rear-drive axle; and, in order to prevent undesirable xe2x80x9cwind-upxe2x80x9d between the front and rear of each driven track, the combined driving and steering torque transmitted by the respective output gear of each orbital steer-drive unit is connected to a respective left- and right-side torque-proportioning differential which then divides and delivers the combined driving and steering torque differentially between the front- and rear-drive axles of each track. While such known torque-proportioning differentials are used with prior art steer-drives, it should be noted that, with the invention""s preferred steer-drive embodiments (designed to minimize speed reduction), the size of these known differentials may be reduced.
In addition to having fewer and less expensive parts, and in addition to providing a much simpler means for combining steering and driving torque, the orbital steer-drive units of the invention have another important advantage over the prior art: Design variations are greatly facilitated, since gear speed ratios can be selected throughout a much wider practical range (e.g., 1:1 to 13:1); and, further, throughout this extended range, the physical size of the invention""s orbital drive units (e.g., measured outside diameter) is smaller than the size of a standard reduction gear drive for any particular speed ratio selected. However, in view of the invention""s expressed goal to limit the size and weight of steer-drive systems when used in track-laying vehicles, selection of gear ratios equal to, or less than, 3:1 are preferred.
Thus, the components of our orbital steer-drive (as well as the components of the accompanying torque-proportioning differentials used in embodiments designed for track-laying vehicles) can be smaller, lighter, and more compact; and almost all of the speed reduction desired between the engine and each of the vehicle""s propulsion elements can be realized between the output axles of the orbital drive units and the propulsion elements or, in the case of tracked vehicles, between the output axles of each torque-proportioning differential and the front- and rear-drive axles of each respective track. For instance, where the steer-drive is designed for a tracked vehicle intended for use at highway speeds, the final connection between each differential and the drive axles for each track includes a further speed reduction selected so that, when the transmission is operating at 1:1 with the vehicle""s engine, there will be an overall speed reduction ratio of approximately 4-5:1.